Traditionally, zirconium boride and composites thereof, such as zirconium boride/silicon carbide composites, have been fabricated by a hot pressing process. Mixtures of zirconium boride and silicon carbide powders are placed in a pressure vessel and are subjected to elevated pressures while heated to high temperatures, typically in an inert atmosphere or under vacuum. Alternately, ZrB2/SiC composites may be formed by reaction hot pressing precursors such as metallic Zr, Si powders and boron carbide (B4C) powder (instead of SiC and ZrB2 powder precursors). In either technique, the lack of self-diffusion and low driving forces for sintering and densification inherent in the materials is compensated for through the application of high pressures during the sintering step. The high pressures applied to the sintering body contribute sufficient forces such that substantially complete densification of the sintering body may be achieved.
Typically, substantially dense composite bodies are formed as follows. First, the raw material powders are blended and then loaded into a simple geometrical model, such as a graphite die, where the blended raw materials then undergo heating and pressing simultaneously. Although hot pressing is not required per se for the sintering of ZrB2/SiC composites, sintering without the application of elevated pressures results in weak bodies characterized by densities only about 90 percent of theoretical and having poor thermal and mechanical properties. Therefore, the densified bodies so produced are limited by the constraints of the hot pressing die to simple shapes and moderate sizes. Further, hot pressing techniques require expensive hot pressing facilities and provide a slow rate of production. Moreover, the bodies produced by hot pressing techniques are simple and unfinished, thus typically requiring further diamond machining in order to produce a finished end product. Such machining adds considerable time and financial cost.
In the hot pressing processes, the attendant high pressures are necessary to provide sufficient driving force for substantial densification to occur, since the mixed ZrB2 and SiC powders alone lack sufficient self-diffusion characteristics when heated to sintering temperatures. The use of high sintering pressures addresses this problem by providing an externally generated driving force to the system, but also adds complexity and cost to the fabrication of ZrB2/SiC bodies. Further, the application of high pressure adds inherent geometrical constraints that limit the bodies so formed to simple geometric shapes. Thus, there remains a need for a means of fabricating and sintering ZrB2/SiC bodies having complex shapes at ambient pressures. The present invention addresses this need.